1. Field of the Invention
The present invention relates to a driving circuit which drives a light emitting diode(s), and in particular to a light-emitting diode driving apparatus which drives a light emitting diode(s) by using AC power supply.
2. Description of the Related Art
In recent years, significant attention is given to light-emitting diodes (hereinafter, occasionally referred to as “LEDs”) as lighting sources. The reason is that LEDs can be driven at low power consumption as compared with filament lamps or fluorescent lamps. LEDs are small, and have shock resistance. In addition, LEDs are less prone to blow out. Thus, LEDs have these advantages. In the case of lighting sources, it is desirable that commercial AC power for home use is used as power supply for lighting sources. LEDs are devices driven by DC power. LEDs emit light only when applied with a current in the forward direction. Also, in the case of LEDs that are currently typically used for lighting use, the LEDs operate on DC power at a forward directional voltage Vf of about 3.5 V. LEDs do not emit light if a voltage applied to the LEDs does not reach Vf. Conversely, a voltage applied to the LEDs exceeds Vf, an excessive amount of current will flow through the LEDs. Accordingly, it can be said that DC power is suitable for driving LEDs.
In the case where LEDs are driven for light emission by using AC power supply, an AC current is typically converted into a DC current by a switching power supply so that the converted current is applied to the LEDs. However, in this construction, circuit elements such as coil and capacitor are required. Accordingly, a large power supply is required. As a result, this construction will be expensive. For this reason, various types of driving apparatuses have been proposed to drive LEDs with a pulsating current which is rectified a current from AC power supply by a diode bridge without using switching power supply.
FIG. 19 shows a circuit diagram of a constant current LED driving circuit which has been developed by the inventors and uses two transistors, for example. The constant current LED driving circuit includes commercial power AP, a rectifying circuit 92 which is composed of a diode bridge, an LED portion 91, and a constant current circuit which includes two transistors 93 and 94 and resistors 95 and 96. The LED portion 91 includes a plurality of LED devices, which are serially connected to each other. In this constant current LED driving circuit, in the case where the sum of Vf values (hereinafter, occasionally simply referred to as Vf) of the plurality of LED devices is low, the loss of the constant current circuit will be large which is caused by the voltage difference between Vf and the peak of rectified pulsating voltage (141 V in Japan). As a result, in this case, the power supply efficiency of the constant current LED driving circuit will be low. On the other hand, in the case where the sum of Vf values is high, the voltage range wil be narrow where the LED portion can be driven. As a result, the LED light ON period will be short in one cycle. In addition, in the case where the sum of Vf values is set high, the number of the serially-connected LED devices is necessarily increased. As a result, while the cost will be high, the LED usage efficiency will be low, in other words, (LED effective power consumption)/(power consumption of LED driven at rated DC current) will be low. FIG. 20 is a graph showing this relationship. In this graph, the horizontal axis indicates the sum value of Vf values of the LED devices which are driven at a constant current. The left-side vertical axis indicates the power supply efficiency and the driving efficiency, while the right-side vertical axis indicates the crest factor. As seen from this graph, the power supply efficiency decrease with the decrease of the sum of Vf values, while the driving efficiency decreases with the increase of the sum of Vf values. For this reason, they are mutually contradictory.
In addition, the constant current LED driving circuit has a light OFF period where the LEDs are brought OFF in synchronization with power supply frequency. This may deteriorate lighting quality. The crest factor (=maximum value/effective value) is well known as objective index. In this case, the light output values are measured to obtain the crest factor. The aforementioned constant current driving circuit shown in FIG. 19 has a crest factor=about 1.7 in the case of the sum of Vf values=120 V. This crest factor is larger than filament lamp, fluorescent lamp and inverted fluorescent lamp, which have crest factors of about 1.05, 1.36 and 1.1, respectively. This means that some people may perceive flicker. In case where the LEDs are lighting a rotating body, if the power supply frequency matches with the rotating frequency, it may perceived that rotating body is stopped even though it rotates (stroboscope phenomenon). To avoid this, it is conceivable that a large-capacitance capacitor is used for smoothing the pulsating current. However, in this case, the power factor may be reduced by a quick charge current flowing into the capacitor. Also, an inrush current may be produced when the power supply is connected.
As discussed above, in the case where AC power supply is used, if the LEDs are driven with the pulsating current after rectification, it is not easy to achieve a good balance between power supply efficiency, power factor, LED usage efficiency and light output crest factor. As can be seen from FIG. 20, the power supply efficiency and the LED usage efficiency have a tradeoff relationship with each other. For this reason, it is necessary to find an appropriate balance (in other words, to make an appropriate compromise) between the power supply efficiency and the LED usage efficiency. On the other hand, in order to improve the light output crest factor, the light ON period may be increased by discharging the capacitor. In this case, although the LED usage efficiency can be improved, the power factor will be reduced by the charging current of the capacitor.
See Japanese Patent Laid-Open Publication No. JP 2006-147,933 A.
The present invention is devised to solve the above problems. It is a main object of the present invention to provide a light-emitting diode driving apparatus which can surely has both a good crest factor and a good driving efficiency.